The introduction of bone cement in the 1960s was a major breakthrough in orthopedic surgery. It was possible thereafter for the surgeon to replace nearly any damaged joint with implants attached to the bone with cement that fills irregularities of the bone/implant interface and furthermore acts as a filler of larger spaces or voids. Recent estimates indicate that each year in industrialized countries there are 1,000 implants per one million inhabitants. These have mainly involved the implantation of artificial hip joints.
Although the initial results of such implants have been excellent, long term clinical studies have shown loosening of the implant with time. This occurs either at the interface between implant and cement or more commonly at the interface between cement and bone. After about 10 years of service, at least 20% of hip joint implants require revision, which generally involves the insertion of new components.
Such high replacement statistics have led to considerable work to improve implant designs and to improve the bone cements. The latter generally have been formulated from methylmethacrylate (MMA) monomer and polymethylmethacrylate (PMMA) powder. Commercially available bone cements have differed from each other in particle size and composition of the PMMA powder, concentrations of accelerators such as N,N-dimethyl-p-toluidine, or the use of special additives such a radiopaque or X-ray contrast agents, antibiotics, dyes and the like.
As is well documented in numerous medical studies as well as in the patent literature, the cement, which cures in vivo after insertion of the implant, gives rise to problems; the major problem is bone necrosis, often a much as 0.5 cm, caused by the high exotherm at the interface between cement and bone and release of monomeric methylmethacrylate. High exotherm, exceeding 70.degree. C., results from the polymerization or curing taking place in the bone cement following admixing of the ingredients immediately preceding its use with the implant. Unreacted methylmethacrylate is the source of the released monomeric methacrylate, which is highly toxic to bone cells. Moreover, the literature indicated that monomeric methacrylate myy deleteriously affect local blood circulation as well as the blood pressure of the patient.
It also has been observed that with time a membrane is formed at the cement/bone interface. As the membrane increases in thickness the bone becomes thinner and loosening of the implant and secondary dysfunction is observed. Ultimately, revision surgery is required. Although the reasons for this membrane formation are not fully understood at this time, histologic examinations show unspecified inflammatory tissue and it is supposed that the long term release of monomeric methylmethacrylate as well as an accelerator such as N,N-dimethyl-p-toluidine stimulates the development and growth of the undesirable inflammatory membrane.
Set forth below are prior patents that reveal the development of acrylate bone cements, the disadvantages of such bone cement, and recent efforts to overcome these problems; a discussion of the most relevant patents follows: